In general copyright doesn't cover designs for machine parts, except insofar as they have an expressive rather than functional character, in which case they can be copyrighted as sculptural works. But you can still make replacement parts that copy their functional aspects.
If you want a monopoly on copying a machine part, you need to apply for a patent, not a copyright. Among other differences, the patent only lasts 20 years, and doesn't exist automatically but must be granted explicitly after a long litigation process — the vast majority of machine parts are not patented.
If you want a monopoly on copying a machine part, you need to apply for a patent, not a copyright. Among other differences, the patent only lasts 20 years, and doesn't exist automatically but must be granted explicitly after a long litigation process — the vast majority of machine parts are not patented.